1. Field of the Invention
This invention relates to an optical scanning system for scanning a spot formed on a scanning surface by a deflector through a telecentric scanning lens. More particularly, it relates to an optical scanning system and a method for adjusting thereof which includes means for detecting a focusing state of the spot on the scanning surface.
2. Description of the Prior Art
In an optical scanning system, wherein the flatness of a scanning surface of a photographic plate, film, etc. is low and wherein there is a possibility for the scanning surface to be shifted forward and backward in the direction of the optical axis, or wherein a high accuracy is required for an image position, a telecentric scanning lens is used so as not to displace the position of an image when a focusing error is occurs.
Since the optical scanning system requires a certain period of time for forming a two-dimensional image, it is impossible to detect the adjusting state by projecting an image as in the case with a non-scanning type imaging system when various optical elements (i.e., lenses, a deflector, etc.) are adjusted in an optical axis thereof, or in a focal point, or in eccentricity.
Therefore, the adjusting work was often performed with difficulties.
Furthermore, in an optical scanning system which requires a high drawing density, it is necessary to use a scanning lens having a small F-number in order to reduce the diameter of the spot on the scanning surface. For example, heretofore, the spot diameter required for drawing a pattern of a printed circuit board on a dry plate, film, etc. was a minimum of approximately 30 .mu.m. However, the spot diameter is a minimum of approximately 5 .mu.m for one in which a high accuracy is required, as in the case for drawing lead frame for an IC, etc. or for an apparatus in which a high density is required, as in the case for writing onto a liquid crystal display, etc.
In the case of a Gaussian beam, if the spot diameter is represented by S, the F-number of the beam focused by a scanning lens is represented by FNO, the wave-length is represented by .lambda. and the depth of focus is represented by f.sub.d, the following relations are obtained: EQU S=(4/.pi.).multidot..lambda..multidot.FNO
., EQU f.sub.d =(4/.pi.).multidot..lambda..multidot.(FNO).sup.2
Therefore, in the case where .lambda.=488 nm, in order to make S=30 .mu.m, F-number is about 1:50, and the depth of focus at this time is f.sub.d =.+-.1.55 mm. Therefore, even if the flatness of the scanning surface is .+-.1 mm, the spot diameter is hardly changed and no substantial affect is given to the drawing performance.
However, in order to reduce the spot diameter to S =5 .mu.m with the same .lambda.=488 nm, F-number must be about 1:8, and the depth of focus at this time becomes f.sub.d =.+-.0.04 mm. Consequently, even when the scanning surface has a minor irregularity (or waviness) of about 0.1 mm, the spot diameter is largely changed to deteriorate the drawing performance.
Such inconvenience as just mentioned arise not only by irregularities or waviness of the scanning surface, but also by the inclination or displacement of a lens and the curvature of field of the scanning lens.
The prior art of telecentric scanning lens is disclosed in Japanese patent early laid-open publication No. Sho 60-123815. The telecentric f.theta. lens disclosed in this publication has a positive power as a whole, a negative power of the first group is struck out by three positive lenses, a meniscus lens being disposed in such a fashion as to face its concave surface toward the image field side having a small power in order to obtain a total balancing.
However, the telecentric f.theta. lens described in the above-mentioned publication is small in radius of curvature of three positive lens group on average, and spherical aberration due to insufficient correction occurred at these lenses and the large Petzval sum become more difficult to be corrected by negative lenses of the final group, particularly as the F-number becomes smaller. Therefore, in order to restrain the affection of the spherical aberration and the curvature of field, it is obliged to limit the F-number to approximately =8.3 for practical use.
In order to increase the volume of information on the scanning surface, it is required that F-number of the lens is made small in order to make small the spot diameter and that various aberrations are restricted to small values.
However, it is difficult for the above-mentioned lenses to meet these requirements. Also, in general, as the F-number becomes smaller, it becomes more difficult to correct the aberration within a wide range of scanning angles and therefore, it is difficult for the f.theta. lens, for which high accuracy was heretofore required, to take the range of scanning angle wide.